In many residential applications elevator cars and their hoisting systems are dimensioned far (a) maximum planned traffic capacity or maximum number of persons, (b) size of floor area to satisfy occasional large furniture removal and/or wheelchair access. Particularly in counterweightless or counterweightfree elevators, that is mainly hydraulic elevators and drum elevators, this leads to bulky motors and large fuses which can cause a lot of problems, especially when installing new elevators in older buildings or modernizing or upgrading old elevators. Naturally bulgy motors and large fuses and associated high current electric cables also cause higher costs.
However, in majority of trips the elevator carries typically less than 30% of the rated load. Approximately half of the trips, there are no persons in the elevator car (see FIG. 1 a hypothetical usage curve of a counterweightless elevator).
In the traction sheave elevators., the counterweight is generally dimensioned on the weight of the car and half the payload. This means that energy corresponding to the weight of the car is saved, both when the car is traveling full and empty. However on empty down trips, which is common in residential elevators, the hoisting system requires its maximum power, as it has to be able to lift the net difference between the counterweight and the unloaded car. This leads to unnecessary energy consumption. U.S. Pat. No. 5,984,052 discloses a counterweight elevator system which includes a control system that determines the amount of load of the car, and that determines the operating speed profile of the car based upon the amount of load in the car. In a particular embodiment, the control system includes a load weighing device and uses the weight of the car to determine the selection between two operating speed profiles: a normal operating speed profile and a reduced operating speed profile. The control system compares the measured live load to a pre-selected threshold, such as the car weight plus twice the percentage balancing multiplied by the rated full load of the elevator system. If this threshold is exceeded, then the reduced operating speed profile is selected. In this way, reduced balancing may be used. The selected percentage balancing may be determined empirically or estimated by taking into account the building size, usage and other operational characteristics. Thus, in U.S. Pat. No. 5,984,052 energy can be saved by dimensioning the counterweight based on less than half the payload and by reducing the speed of the hoisting system when the car is loaded closer to full capacity. This kind of a reduced counterweight system is difficult to realize in practice.
In many cases the counterweightless hydraulic or drum driven or screw driven or chain driven elevators are used because they offer certain advantages for example with respect to shaft space efficiency. A prior art solution to reduce the hoisting motor size in counterweightless elevators is to dimension the motor smaller than normally by a certain factor and limit the starts per hour. However, this means that the motor still needs to be dimensioned at approximately 70% of full capacity. On empty up trips this means that the motor consumes energy to carry the weight of the car and almost the full payload.